Carbon materials, such as carbon/carbon composites, have a wide variety of uses in modern industry, especially in the areas of aerospace and aviation. However, these materials are subject to degradation, especially oxidation, at elevated temperatures. The resulting carbon monoxide and carbon dioxide do not offer any protection from oxidation for the carbon substrates. As a result, these carbon substrates are in need of a protective coating or substance which would prevent oxidation and degradation at elevated temperatures. A common coating for high temperature applications in an oxidizing environment is silicon carbide (SIC), like that disclosed in U.S. Pat. No. 4,830,919. This coating intercepts the incoming oxygen, producing silicon dioxide (SiO.sub.2) through the following reaction: EQU SiC(s)+2O.sub.2 (g).fwdarw.SiO.sub.2 (l)+CO.sub.2 (g)
wherein:
s=solid phase
g=gaseous phase
l=liquid phase
Above 1600.degree. F. the liquid SiO.sub.2 wets the underlying SiC coating of the carbon substrate, forming a thin, continuous film. SiO.sub.2 is known in the industry to be an excellent oxygen diffusion barrier. Therefore, the success of these systems to protect carbon substrates from oxidative attack is based on an adherent SiC coating and the formation of a continuous SiO.sub.2 film.
Cracks form in the SiC coating at temperatures above about 900.degree. F. due to the thermal expansion mismatch between the protective coating and the carbon substrate, and below 1600.degree. F. the SiO.sub.2 protective layer offers little protection from oxidation due to the discontinuous nature of the film. The formation of sufficient liquid SiO.sub.2 to protect the SiC coating and the carbon substrate occur only above 1600.degree. F. At temperatures in the range of 1600.degree.-3200.degree. F., the cracks in the SiC coating tend to be filled with liquid SiO.sub.2. Thin cross sections are required for advanced aerospace structures, such as elevons, ailerons, body flaps, etc. Thus, thin, uniform coatings of SiC with a controlled microstructure are required in order to maintain material structural integrity.
Thus, a need exists for a composition of matter and a process for forming a protective coating on carbon substrates to protect such from oxidation and degradation at temperatures lower than 1600.degree. F., as well as at temperatures above 1600.degree. F. A need exists also for a uniform coating with a controlled microstructure such that there is material structural integrity.
Accordingly, the present invention discloses a composition of matter and a process for forming a protective coating which has the technical advantage of protecting carbon substrates from oxidation and degradation at temperatures between 900.degree.-1600.degree. F. as well as at temperatures above 1600.degree. F. up to 3200.degree. F. The present invention discloses a two-step conversion coating process which produces a protective coating with a uniform, controlled thickness and a microstructure with Si and B.sub.4 C particles distributed throughout. The boron of the coating can be in the form of B.sub.4 C, which forms B.sub.2 O.sub.3 when oxidized at 900.degree. F. or above, which in turn modifies any solid SiO.sub.2 formed on the coating film, to produce a liquid B.sub.2 O.sub.3 /SiO.sub.2 mixture. This liquid mixture fills in the cracks in the SiC coating that occur at temperatures lower than 1600.degree. F., thus offering a significant amount of protection from oxidation at temperatures in the range of about 900.degree. F. to about 1600.degree. F.
Further, a controlled, uniform thickness is desirable for the efficacy of a protective coating since the initial SiC base coating depletes at high temperatures due to the formation of gaseous SiO from solid and/or liquid SiO.sub.2. The thicker the coating, the better the oxidation performance; but on the other hand, as the coating thickness is increased, the composite strength is inevitably decreased. In light of such, another technical advantage of the present invention is a thin coating of a uniform, controlled thickness, containing finely dispersed Si and B.sub.4 C dispersed at least generally throughout the SiC coating. Thus, the present invention offers excellent oxidative protection while not sacrificing mechanical and structural integrity of the carbon substrate.
Also, the SiC coating needs to be dense so that the formation of the protective SiO.sub.2 film becomes continuous. If the SiC coating is too porous, there can be breaks in the SiO.sub.2 film, resulting in oxygen ingression to the carbon substrate. However, if the coating is overly dense, the evolution of CO.sub.2, which occurs during the oxidation of SiC to form SiO.sub.2, could cause coating failure. Thus, there needs to be some porosity to allow the gaseous CO.sub.2 to evolve. With the addition of boron, the present invention provides yet another technical advantage with an excellent balance between porosity and density by consolidating or filling in the cracks in the initial SiC coating that occur at temperatures below 600.degree. F. while at the same time also allowing a sufficient amount of CO.sub.2 to evolve.